North Korea billed the United States $2 million for the medical care of a U.S. college student who was detained in Pyongyang in 2016 and returned to the United States comatose more than a year later, according to reports in The Washington Post.

Sources told the Post the main U.S. envoy sent to retrieve the unresponsive Otto Warmbier signed an agreement to pay the medical bill at the direction of President Donald Trump. The bill went to the Treasury Department, but it was not clear whether the administration paid the invoice

'We do not comment on hostage negotiations, which is why they have been so successful during this administration,' said White House press secretary Sarah Sanders in a message on social media.

As recently as Sept. 30, Trump asserted that his administration paid 'nothing' to get American 'hostages' out of North Korea, the paper reported.

Warmbier was detained in January 2016 for allegedly stealing a banner in a North Korean hotel. Warmbier was 21 years old when he was arrested. After appearing in court and delivering an emotional apology that was videotaped and widely shared on social media, he was convicted in March 2016 and sentenced to 15 years of hard labor in a North Korean prison.

The mainland China travel company that arranged Warmbier's trip, Young Pioneer Tours, specializes in 'destinations your mother would rather you stay away from,' according to its website. It describes itself as 'safe and fun.' Photos from the company's website and Facebook page show selfies of happy, smiling, young Westerners in Pyongyang.

North Korean officials said Warmbier fell into a coma the night he was sentenced in March 2016, the paper stated. Doctors have not identified the cause of his brain damage, and say they did not see evidence of his being beaten. Warmbier died six days after he was flown back to the United States.

At their last meeting in Hanoi in February, President Trump said he accepted North Korean leader Kim Jong Un's claim not to have known what had happened to Warmbier in prison, despite the case being extraordinarily sensitive.

'I will take him at his word,' Trump said.

Warmbier's father, Fred, said he was unaware of the hospital bill. He told the Post it sounded like a 'ransom' for his late son.

Following Otto Warmbier's sentencing, the North Koreans did not tell U.S. officials until June 2017 that he had been unconscious for 15 months. The paper said news of Warmbier's condition sparked a frantic effort to get him home. The effort was led by the State Department's point man on North Korea at the time, Joseph Yun, the paper reported.